1. Field of the Invention
This invention relates to a filter characteristic regulating apparatus for regulating a characteristic frequency to a desired frequency in a filter device capable of regulating a characteristic frequency, and more particularly to a filter characteristic regulating apparatus having a simple construction that can be mounted onto one semiconductor chip together with the filter device, and to a method therefor.
2. Description of the Related Art
It has been desired in recent years to mount a continuous time filter, that is used for selectively adopting or rejecting weak signals, to a semiconductor integrated circuit with the improvement in the integration density of the semiconductor integrated circuit. Such a filter device has a characteristic frequency representing its characteristic feature. For instance, the center frequency is the characteristic frequency for a band-pass filter and the cut-off frequency is the characteristic frequency for a low-pass filter and a high-pass filter.
Generally, the characteristics of circuits mounted to the semiconductor integrated circuit fluctuate with fluctuation of a production process or the operating condition. Therefore, the characteristics of the filter device mounted to the semiconductor integrated circuit fluctuate, too, depending on the production process and the operating condition.
To keep the characteristics of the filter device mounted to the semiconductor integrated circuit to a desired value, a filter characteristic regulating apparatus for automatically regulating the characteristic frequency of the filter device to a desired frequency becomes necessary.
FIG. 1 is a block circuit diagram showing the filter device mounted to the semiconductor integrated circuit and the filter characteristic regulating apparatus according to the prior art. In FIG. 1, reference numeral 1 denotes a filter device capable of regulating a characteristic frequency thereof, and reference numeral 2 denotes a filter characteristic regulating apparatus according to the prior art. This filter characteristic regulating apparatus 2 includes an oscillator 3 capable of regulating an oscillation frequency, an oscillation frequency measuring instrument 4 for measuring the oscillation frequency of the oscillator 3, and a controller 5.
The controller 5 supplies, to the oscillator 3 and to the filter device 1, a characteristic controlling signal such that the oscillation frequency of the oscillator 3 becomes a constant frequency, on the basis of the measurement result of the oscillation frequency of the oscillator 3 by the oscillation frequency measuring instrument 4, and controls the oscillation frequency of the oscillator 3 as well as the characteristic frequency of the filter device 1.
In other words, the filter characteristic regulating apparatus 2 according to the prior art designs the oscillator 3 so that the oscillation frequency of the oscillator 3 and the characteristic frequency of the filter device 1 have a known correlation, and regulates the characteristic frequency of the filter device 1 to a desired frequency by controlling the oscillation frequency of the oscillator 3 to a constant oscillation frequency.
As described above, the conventional filter characteristic regulating apparatus 2 regulates indirectly the characteristic frequency of the filter device 1 by controlling the oscillation frequency of the oscillator 3, and such a filter characteristic regulating method is generally referred to as a "master-slave system". In this system, the filter characteristic regulating apparatus 2 is the master and the filter device 1 is the slave.
Incidentally, a relative value of each device constituting a semiconductor integrated circuit does not generally attain a design value but involves a certain variance (a few percent in a CMOS process). Therefore, if the values of the individual elements fluctuate in the conventional filter characteristic regulating apparatus 2, the actual correlation between the oscillation frequency of the oscillator 3 and the characteristic frequency of the filter device 1 becomes different from the correlation as designed.
The errors of the oscillation frequency of the oscillator 3 and the characteristic frequency of the filter device 1 from the design values due to relative variation of the individual element values correspond to the values that are inherent to each device. Therefore, it is difficult to estimate and correct such a value at the time of design, and such estimation and correction have not been done.
Under such circumstances, if the actual correlation between the oscillation frequency of the oscillator 3 and the characteristic frequency of the filter device 1 deviates from the design values in the conventional filter characteristic regulating apparatus 2, the characteristic frequency of the filter device 1 deviates from the desired frequency, and selectivity of the filter device 1 to the input signals is deteriorated.
When a high precision filter device is necessary, therefore, a filter device that is in advance regulated to high accuracy must be provided outside the semiconductor integrated circuit, and this hinders the reduction of the size and the cost of the overall apparatus requiring such a filter device.
Taking the problems described above into consideration, the prior art technology "Integrated Filter Circuit and Regulating Method thereof" disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-114836 is directed to provide a high precision active filter that can be incorporated into the semiconductor integrated circuit. However, the technology disclosed in claim 6 of this Japanese Unexamined Patent Publication (Kokai) No. 5-114836 first inputs a measuring signal to a filter device so as to obtain a response measuring signal, subjecting this signal to frequency transformation such as Laplace transformation and calculating a transfer characteristic of the filter. Next, secondary state variables Q and .omega.o of a state-variable filter are calculated from this transfer characteristic and are then compared with state variables that are set in advance to determine an error between them. This error is fed back to the filter device in order to regulate the filter highly precisely.
As described above, the technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-114836 requires an extremely complicated process such as a Laplace transformation for determining the transfer characteristic of the filter. However, to accomplish such a complicated processing function by a semiconductor device such as an LSI, at least a plurality of LSIs are necessary. Therefore, it is almost impossible to incorporate the circuit accomplishing this regulating method with the filter device into one LSI.
As a matter of fact, Japanese Unexamined Patent Publication (Kokai) No. 5-114836 constitutes filters 42 to 44, D/A converters 45 to 50 and a ROM 57 inside the object apparatus (for example, one LSI), and accomplishes a circuit for an arithmetic operation, inclusive of a CPU, and a circuit for generating a regulating signal, by external circuits, as can be seen clearly from FIG. 3 of this reference.